1. Field of the Invention
This invention relates to a color signal emphasizing/de-emphasizing circuit for a color printer or a color video display. More particularly it relates to a color signal emphasizing/de-emphasizing circuit capable of reproducing the original colors of the target image with high fidelity by correcting a hue or saturation thereof or to a circuit capable of changing only the specified color without exerting any influence on other colors.
2. Description of the Related Arts
According to the conventional color signal correction circuit, it is possible to produce an output of the target image on a CRT or on a paper whenever it is necessary. This is interpreted as an electrical signal by a TV camera or an image scanner. It is further stored in the form of electric data such as the image information.
Although a pixel, or a picture element, of the target image is generally interpreted as brightness information in the existing image processing using a black-and-white signal, in the case of a color image processing, each pixel is interpreted as the three principle color signals.
As is well known, the three principle colors consist of yellow (Y), magenta (M) and cyan (C) in the printing color principle while they consist of red (R), green (G) and blue (B) in the optical color principle employed in the display such as a CRT.
The present invention relates to the color signal emphasizing/de-emphasizing method and apparatus capable of emphasizing or de-emphasizing any desired color when producing an output of an image on a paper or a display based on the three principle colors obtained by scanning the target in every pixel.
To reproduce the colors scanned in every pixel on a paper of a printer or on a CRT with high fidelity, for example in the case of a sublimation type printer, it is necessary to correct the color saturation included in a sublimation type ink. To this end, a color masking method employing the equation 1 as shown below has been used as a signal converting method to obtain Yo, Mo, Co signals required to eliminate color saturation errors from Yi, Mi, Ci signals input to the printer. The equation is expressed as; ##EQU1## where Ci, Mi, Yi denote the original signals and Co, Mo, Yo the emphasized/de-emphasized signals. Moreover, the "aij" (1.ltoreq.i.ltoreq.3, 1.ltoreq.j.ltoreq.3) is an emphasizing/de-emphasizing coefficient and is used to enable an exact color reproduction by correcting the color saturation of ink, which may differ in accordance with the characteristic of the input YMC signals or a printer.
FIG. 5 is a block diagram showing an existing color signal correction circuit to realize the color masking method disclosed in the Japanese Patent Laid Open No. 27262/1988 in which a matrix multiplier 500 is used to obtain the result of the equation 1. In FIG. 5, the original color signals Ci, Mi and Yi are respectively held in latches 540, 541 and 542 according to a signal T1 generated in a timing generator 530 to be delivered to the matrix multiplier 500 as the color signals C1, M1 and Y1. The emphasizing/de-emphasizing coefficient aij (1.ltoreq.i.ltoreq.3, 1.ltoreq.j.ltoreq.3) is given to the matrix multiplier 500 from a coefficient register 560. FIG. 6 is a block diagram showing a detail of the matrix multiplier 500 shown in FIG. 5. The input color signals C1, M1 and Y1 are respectively fed into multipliers 501 through 503, 504 through 506, and 507 through 509. Further, a signal T2 generated in the timing generator 530 is delivered to all multipliers 501 through 509. In the multipliers 501, 504 and 507, the input color signals C1, M1 and Y1 are independently multiplied by the coefficients a11, a12 and a13 output from the coefficient register 560 shown in FIG. 5 according to the signal T2. The multiplied signals are then delivered as inputs of 516, 517 to an adder 510 and as an input of 523 to an adder 513. The adder 510 functions according to a signal T3 generated in the timing generator 530 to produce an input 522 of the adder 513. Besides, the adder 513 functions according to a signal T4 generated in the timing generator 530 to produce an output color signal C2. In the same manner, it is possible to obtain not only an output color signal Y2 by the multipliers 503, 506, 509, adders 512, 515 and inputs of 520, 521, 526, 527 but an output color signal M2 by the multipliers 502, 505, 508, adders 511, 514 and inputs of 518, 519, 524, 525 at the same time. The output color signals C2, M2 and Y2 obtained in the manner as described above are respectively held in latches 580, 581 and 582 according to a signal T5 generated in the timing generator 530 shown in FIG. 5 to be output as emphasizing/de-emphasizing signals Co, Mo, Yo.
According to the existing color signal correction circuit employing the color masking method, a color matching of a printer is performed by changing the emphasizing/de-emphasizing coefficient "aij" if a slight variation of the characteristic of the original signals Ci, Mi, Yi or the color saturation included in ink of a printer occurs. However, it is quite difficult to foresee what kind of color will be reproduced when the emphasizing/de-emphasizing coefficient "aij" is adjusted by the coefficient register 560 shown in FIG. 5. Another problem of the conventional circuit is that since a hue of the entire printed color changes when the emphasizing/de-emphasizing coefficient "aij" is adjusted, it is impossible to slightly change only a color specified, for example to change a color of flowers to make them more reddish, without exerting any influence on the other colors to be printed on a paper or to be displayed on a CRT. A further problem of the prior color signal correction circuit is that cost of the apparatus increases because it requires a number of such complex multipliers as shown in FIG. 6.
To solve the above described problems, the present invention provides a low cost color signal emphasizing/de-emphasizing circuit which is capable of eliminating color saturation errors included in ink of a color printer and of reproducing the original colors of the target image with high fidelity and of slightly changing only a color specified without exerting any influence on the other colors to be printed on a paper.
As is generally known, all colors printed on a color printer or displayed on a color display are based on a combination of the three primary-colors principle, that is yellow (Y), magenta (M) and cyan (C) in the printing color principle, and red (R), green (G) and blue (B) in the optical color principle. In the present invention, as the terms expressing a combination of these three principle colors, there are used terms of a primary color which is a single one of any principle color, a secondary color which has a combination of two principle colors, and a tertiary color which has a combination of the three principle colors. For example, a primary color depicts a color in which one of the three principle colors, i.e. Y, M and C exists solely; a secondary color is a color in which two of the three principle colors are combined in a set, i.e. YM, YC and MC; and finally a tertiary color in which all of the three principle colors are combined together, i.e. YMC.
A feature of the present invention is that a magnitude of each of the original color signals is classified into these primary, secondary and tertiary colors and then these designated colors are independently multiplied by a predetermined coefficient. On this point, however, in the conventional method, all colors of the original color signals are multiplied by the same coefficient as expressed in the equation 1. As a result, it has been impossible to perform a subtle adjustment of each individual hue by the conventional color correction circuit.
According to the present invention, it becomes possible to perform an adjustment for every hue of all colors independently because the original color signals are classified into three color components, or color designations such as a primary color portion, a secondary color portion and a tertiary color portion. A predetermined coefficient is then respectively multiplied to each of these classified color components.
According to the present invention, a primary color portion is a value obtained by subtracting a magnitude of a second largest signal level from a magnitude of the largest signal level that is obtained by comparing the magnitudes of the three principle colors; a secondary color portion is obtained by subtracting a magnitude of a third largest signal level from the magnitude of the second largest signal level; and a tertiary color portion is obtained by subtracting zero from the third largest signal level, equal to the smallest value of the original color signals.